Fuel systems for aircraft gas turbine engines typically include a means for controlling flow rate, as well as a minimum pressure valve, a fuel distribution valve or valves and a positive shutoff valve. The minimum pressure valve prevents fuel flow to the engine burners until a minimum pressure rise has developed across the fuel pump. This minimum pressure rise is required to provide adequate hydraulic force to insure correct operation of engine servo systems which use pressurized fuel for actuation.
The distribution valve provides approximately equal fuel flow to all the engine fuel burner nozzles. A separate flow exit port is generally provided in the valve for each fuel nozzle or small group of nozzles. The arrangement is designed with the fuel pressure drop across this valve window large compared to the static head of fuel from the bottom to the top of the engine. Accordingly, flow to each burner nozzle is relatively independent of the vertical height location of the nozzle on the engine. If the distribution valve were not present, the nozzles at the bottom of the engine would operate at a higher pressure than those at the top of the engine, particularly at low ratings, and these lower nozzles would therefore receive more fuel. This could result in overheating of the lower burners and a consequent reduction in burner or turbine life.
A fuel reprime function must also be provided in most engine fuel systems. If one aircraft fuel tank runs dry, the fuel pump must be capable of removing the air from the pump inlet line and re-establishing fuel flow after switching to an alternate tank. The air removed from the pump inlet line must either be compressed into the pump discharge line or pumped through the pump discharge line, minimum pressure valve, distribution valve, and fuel nozzles out to ambient atmosphere. Since there may be fuel in the line downstream of the pump, the alternative of discharging the fuel is preferred to that of just compressing the air. In any event the fuel must be moved sufficiently that the pump is receiving fuel and able to pump such fuel.
The fuel distribution valve often contains a positive fuel shutoff feature. This usually requires a minimum pressure difference of approximately 20 psi to open. This pressure is sufficient to prevent boiling of the fuel by keeping the vapor pressure sufficiently high with respect to the engine case ambient temperature following shutdown so that boiling fuel does not expand and enter the burners.
During reprime operation the fuel pump must pump air against the 20 psi valve back pressure. Fuel pumps usually cannot generate sufficient pressure when pumping air and they therefore must use other means such as tank mounted boost pumps to effect reprime.
In some existing fuel systems a solenoid valve operated by the aircraft pilot is opened to pump discharge air overboard during reprime operation. This is undesirable because it requires the pilot to turn the solenoid valve off after reprime has been established. If the pilot forgets to turn off this valve, fuel will be continuously pumped overboard through the open valve.
In most existing fuel systems, a metering valve (flow control valve) is used to control fuel to the engine. This valve typically comprises a spool valve with means provided for actuation and spool translation readout. A separate pressure regulating valve maintains the pressure differential across this valve constant. Therefore, the stroke of the metering valve spool can be related to the open flow area of the valve with this area being proportional to fuel flow. The calculated fuel flow value can be used for engine control and managing aircraft fuel resources.
It is possible to dispense with the metering valve and instead modulate pump speed for the purpose of controlling flow. With such a system some alternate means for measuring fuel flow is desirable for use as a feedback element in controlling pump speed. Present fuel flow meters are either unacceptably expensive, susceptible to failure during exposure to fuel contamination, or too large for aircraft installation.